British light-heavyweight Joshua Buatsi secured a victory yet unimpressive start to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a judges' verdict in a tight 10-round contest at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective individual punches in a match where the approaches failed to mesh, with excessive holding disrupting the action.
Two judges scored it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best performance but what defines champions? They succeed," Buatsi stated in the arena as he was met by boos from the spectators.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in last winter - to claim a 20th career victory.
Various analysts believed the challenger, thirty-one, should have been awarded the victory but the contender experienced the second defeat of his 28-bout career.
The Champion Secures Triumph in Dull Encounter
Buatsi's progress has slowed in recent years through management issues and long spells of lack of fights.
Once tipped as the rising talent of the British fight scene, he now stands trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under fresh management.
He opened tentatively, with neither fighter ready to be aggressive in a cagey early stages.
The challenger, whose sole prior loss came at super middleweight against his former opponent in 2022 after breaking his hand mid-fight, was making just his second fight at the higher weight class.
By the middle rounds, Parker had kept it close, causing problems for Buatsi, who utilized a solid defense to edge the better stanzas.
The crowd's patience diminished because of frequent holding, with the challenger slipping to the canvas several times during close exchanges.
The underdog closed well, but several stanzas were hard to judge. Nevertheless, ex-champion the boxing analyst - working with Dazn - had it strongly leaning for Parker, giving Buatsi just only one stanza.
The victor moves on, but with the presence of hard hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still appears challenging.
Hatton Tribute Fuels All-Manchester Fight
In the secondary fight, Lyndon Arthur overcame Bradley Rea by divided decision in a outstanding local rivalry battle to win the European light-heavyweight championship.
The venue became quiet for a deeply poignant tribute prior to the bout as a bell ceremony tolled in memory of the city's native Hatton, who passed away aged 46 in last month.
It was the premier televised match card in the urban center since Hatton's death. As the last bell ended, the silence transitioned to a roar of "only one Hatton".
The fighting during the bout equaled the emotion. The eventual winner was sent to the canvas by a powerful punch squarely on the face in the early round, rising shakily to his feet on the eight count.
But the red-colored shorts of the fighter soon pressed ahead again. On the opposite side, Rea - in rival City blue - was injured early, heavy swelling developing under his eye as the duo traded punishing blows.
The challenger, 27, landed power punches but the champion, thirty-four, fought fire with fire with stinging uppercuts as the action rarely dropped over twelve rounds.
The victor narrowly won the result with judges scorecards of draw, narrow lead and 115-112, but the two athletes were persistent and brave, exactly what their hero the legend embodied.
